1. Technical Field
The invention relates to digital amplitude modulated (AM) radio transmitters. Specifically, the invention discloses an apparatus and method for protecting a radio transmission system from off-frequency exciter generated carrier signals.
2. Description of the Prior Art
Although the present invention may be used in any digital AM transmitter, it is particularly useful in a hybrid analog-digital system such as an in-band on-channel broadcasting system (IBOC) or a high definition radio system. Such a system allows the simultaneous broadcast of both an analog amplitude modulated signal and a digital signal on the same channel assignment as an existing AM broadcasting allocation. In such a broadcasting system, a second signal, containing digital data, is split into a number of carriers, which are positioned in the sideband frequencies of the existing AM signal. These carriers are selected and modulated carefully to avoid interference with the original AM signal. For example, the carriers may be encoded to be orthogonal to the AM signal.
FIG. 1 illustrates a simplified version of a prior art digital AM radio transmitter 10 that may be used in a hybrid analog-digital broadcasting system. The system receives analog audio input at an analog/digital converter 12. The analog/digital converter converts this analog audio signal into a digital signal. The digital signal is combined with inputted digital data at a multiplexer 14. In the illustrated example, the signal is multiplexed via an orthogonal frequency division multiplexing technique.
The signal is then subjected to a Fourier Transform at block 16, resulting in a signal divided into two components in quadrature. These components are modulated at a modulator in quadrature 18 with the AM carrier signal. The resulting signal is filtered by a broad bandpass filter 20 to remove any undesired portions of the signal. The signal is then passed to an amplifier 22 where it is amplified and then transmitted at a radio antenna 24.
When a modulated signal varies from its expected carrier frequency by more than a small margin, it is possible that the signal may damage the transmitter. The output portion (network) of a transmitter (i.e. blocks 22 and 24) is tuned to the expected carrier signal frequency. Significant deviations from that frequency can result in high levels of reflected signal power from the input of the output impedance matching network, where it is least likely to be anticipated or protected against. Reflected power is expected to be seen only at the output of the impedance network typically, and reflected power detectors which are included in the typical transmitter system only look for energy reflected from the load; it will not be able to detect energy reflected from the input to the impedance matching network back into the amplifier(s) If the amount of reflected power is sufficiently large, damage to the amplifier modules can result.
In the prior art transmitter illustrated in FIG. 1, a broadband filter, typically an analog filter, is used to attenuate signals that deviate from the expected frequency for the transmitter. While this approach will attenuate signals that vary widely from the expected frequency, the filter, by necessity, covers a fairly large range of frequencies. Thus, a signal may be close enough to the expected frequency to pass through the filter, but still vary from that frequency sufficiently to cause damage.
Likewise, merely monitoring the level of reflected energy is not effective in protecting the transmitter. Reflected power is typically monitored after the matching network within the transmitter, which may or may not show reflected power at that point depending on the load impedance. If the load were a broadband dummy load, for example, the reflected power would be close to zero since it presents a fifty Ohm impedance across a wide bandwidth.
To avoid the difficulties of measuring this reflected power, it would be preferable to ensure that the reflected power is not generated at dangerous levels. Accordingly, it would be desirable to verify the frequency of the carrier signal prior to amplification and attenuate any off-frequency signals.